4,5, or even ten screens all for charts mostly. If you are day trading, you do not want to flip back and forth from page to page. You want everything there in front of you. The more monitors you have, the more markets you can follow in real time. There is a limit to this however. 6 monitors is about all I can keep track of.

if you just trade one market, like many people here who only trade ES, you still may want multiple monitors as other markets have an effect also such as the dollar, crude, bonds, etc.